[libcxx-commits] [libcxx] [libc++] Fix UB in <expected> related to "has value" flag (#68552) (PR #68733)

[Louis Dionne via libcxx-commits]

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https://github.com/ldionne approved this pull request.

This LGTM once comments have been applied. As soon as the comments have been applied, @jiixyj please ping us so we can merge this. Then we will need to cherry-pick this to `release/17.x`.

@tru Heads up about this patch. Basically, we discovered an important bug in `std::expected` and fixing it requires breaking the ABI. So we really absolutely need to make this change within the LLVM 17 release (which is the first one where we shipped `expected` non-experimentally), otherwise the ABI break will be larger and more painful. Do we have a process for breaking the ABI between minor LLVM releases (i.e. between LLVM 17.0.3 and LLVM 17.0.4)?

Thanks @jiixyj for the work on this patch and for everyone who chimed in. This is a terrifying bug.

https://github.com/llvm/llvm-project/pull/68733